Ultrasound imaging systems detect and display Doppler shifts of returning echoes. The Doppler shifts represent blood flow patterns. Spectral Doppler is one known ultrasound technique to display blood flow patterns associated with a point in a body.
In spectral Doppler, an ongoing series of Doppler spectra are displayed. Each spectrum represents a point in time and a range of frequencies of the spectrum. The frequencies are proportional to blood flow velocities. The signal strength of each frequency is displayed on the spectrum as a brightness. Generally, certain frequencies in the spectrum are due to noise and not blood flow. These noise related frequencies typically have a small amplitude.
Such an ongoing series of Doppler spectra can be used to measure blood flow characteristics. Many of these characteristics depend on the maximum blood flow velocity throughout the heart cycle. Conventionally, the operator hand-traces maximum velocity on a frozen Doppler strip, or spectral display. For each spectrum, the operator manually determines the highest velocity not caused by noise. Various parameters are then calculated from the maximum velocity curve, such as pulsivity index.
Some ultrasound systems automate the maximum velocity curve determination. Typically, an algorithm is applied to each spectrum to obtain the highest frequency signal not related to noise. Different algorithms have been developed for detecting maximum flow in the presence of noise. For example, Hatle et al., "Doppler Ultrasound in Cardiology," 2.sup.nd Edition, demonstrates the use of a multiple moving windows to determine a maximum frequency for each spectrum.
D'Alessio, "Objective algorithm for maximum frequency estimation in Doppler spectral analyzers", Med. & Bio. Eng'g & Computing, 1985, pgs. 23, 63-68, demonstrates another technique for determining the maximum frequency for each spectrum. D'Alessio obtains the energy associated with the highest negative or positive frequency signal. The energy is multiplied by a scale factor to create a minimum signal energy threshold. The highest positive or negative frequency with an energy above the minimum signal energy threshold is selected as the maximum frequency. The maximum frequency is converted to a maximum velocity. A maximum velocity curve is obtained by repeating the determination of the maximum velocity for each of multiple ongoing spectra.
The algorithms discussed above have one or more constants. The maximum velocity is a function of the constants. The constants are either set by the design of the ultrasound machine or are set by the operator as part of a set-up function.
One ultrasound machine may be used for many applications, such as examination of different disease states or locations in a body. However, none of the ultrasound systems for obtaining and displaying ultrasound data is entirely satisfactory for each application. It is therefore desirable to provide an improved ultrasound system for obtaining and displaying ultrasound data.